Machinery and equipment operated in harsh environments are often subject to accelerated corrosion rates which, if not monitored or controlled, can result in premature aging and eventually failure of the machinery and equipment. Stainless steel alloys have been developed to reduce or inhibit general corrosion and/or the onset of localized corrosion such as crevice and/or pitting corrosion. Components made from the steel alloys may be inspected for corrosive damage and repaired or replaced as necessary based on observed or extrapolated corrosive damage.
Electrochemical polarization scans may also be performed on components made from steel alloys to identify the components' resistance to corrosion. For example, FIG. 1 provides an exemplary polarization plot or curve that correlates the corrosion potential (Ecorr) across the surface of a passivated steel alloy component to the onset of particular forms of corrosion. As shown in FIG. 1, the passive oxide film may protect the surface of the steel alloy component by reducing the increase in the current density (I), and thus the general corrosion rate, as the corrosion potential (Ecorr) increases across the surface of the steel alloy. Eventually, the corrosion potential (Ecorr) reaches the pitting breakdown potential (Eb), at which point the current density (I) increases dramatically, resulting in pitting corrosion, crevice corrosion, and/or other forms of localized corrosion on the surface of the steel alloy component. The localized corrosion may continue to occur until the corrosion potential (Ecorr) decreases below the repassivation potential (Erp).
Polarization scans are often performed in laboratories having the capability to accurately and precisely measure voltages and currents. For example, a sample of a steel alloy component may be connected to a variable power supply and a sensor and immersed in an electrolyte. As the voltage across the sample is varied, the current induced across the sample may be measured and graphed to produce a characteristic polarization curve for the particular steel alloy component being tested. The characteristic polarization curve for the particular steel alloy component may then be used to predict the rate and/or onset of various forms of corrosion for components containing the steel alloy.
The characteristic polarization curve, however, does not typically reflect any changes in corrosion rates attributable to pollution or contaminants deposited or precipitated onto the steel alloy component during operations. For example, ambient air flowing into a compressor often includes various amounts of moisture, salts, acids, and other pollution and contaminants that may deposit or precipitate onto various components inside the compressor. The build-up of pollution and contaminants on the components results in an environment conducive to increased levels of general, crevice, and/or pitting corrosion on the components that are not typically reflected in the characteristic polarization curve.
To adjust the characteristic polarization curve for actual conditions experienced during operations, several samples must be taken after operation, analyzed, and then recreated as best as possible in a laboratory for testing to determine what effect, if any, the pollution or contaminants have on the polarization curve. The testing generally requires some amount of disassembly and reassembly to obtain a suitable sample of the component, during which time the component may not be available for operation. As a result, a system and method for performing a polarization scan in situ would be useful in reducing the disassembly and reassembly associated with obtaining a suitable sample.